1. Reef knot  to join two ropes together.
2. Bowline  to fasten a rope to a strong point.
3. Clove hitch  to fasten a rope to a pole (or such like) quickly and you can easily lengthen or shorten the rope.
4. Figure of eight and on the bite  used as a stopper knot (never tie an overhand knot otherwise known as a thumb knot), or to put a loop in the rope.
5. Highwaymans hitch  has two useful features, first, once tied onto a strong point it has a load bearing end, and a quick tug of the other end releases the rope. Secondly, the knot is tied using a bite so you can tie it and release it without having an end.

With just these 5 knots you can do almost everything.
Each knot is easy to undo even if a lot of force has been put on them.